1. Field of the Invention
The present invention relates to a method for analyzing a condition of at least a partial volume of a catalytic converter through which a gas mixture flows, in particular for monitoring an operation of a catalytic converter placed downstream of an electronically controlled internal combustion engine. The main fields of use for the invention are catalytic converters as well as so-called precatalysts disposed next to engines of passenger cars with Otto engines. In connection with increasingly more stringent world-wide laws for the reduction of noxious matter emissions from motor vehicles with internal combustion engines, it also becomes increasingly necessary to monitor the operation of a catalytic converter installed downstream of an internal combustion engine continuously or at intervals in order to obtain information regarding its condition, its effectiveness and, if possible, its remaining service life. Such monitoring is to be performed at opportune times in the course of operation and is therefore identified by the abbreviation OBD (on-board diagnosis).
Numerous methods for monitoring the operation of a catalytic converter which are known from the prior art, make use of various physical and chemical effects. First, it is possible to measure the oxygen content of the exhaust gas ahead of and behind the catalyst by means of so-called lambda probes. In the process it is possible to draw conclusions regarding the operation of the catalytic converter from a reduction in oxygen content between two probes. Another physical principle makes use of the fact that the processes occurring in the catalytic converter are exothermal reactions, which release heat energy. In the simplest case it is therefore possible to draw conclusions regarding the effectiveness of the catalytic converter from the temperature difference between temperature sensors between which at least a large portion of the catalytically active volume is located. Such a method and the associated device are described in Published International Application WO 91/14855, corresponding to U.S. Pat. Nos. 5,255,511 and 5,307,626, for example. Suitable sensors by means of which temperature measurements can be performed in a catalytic converter are described in that reference and also in Published International Application WO 93/05284, corresponding to U.S. Pat. No. 5,474,746. In principle it is necessary in connection with temperature measurements in the exhaust system of an internal combustion engine to distinguish between measurements of the gas temperature and measurements of the structural temperature of the catalytic converter. The structure of a catalytic converter supporting the catalytically active mass on its surface absorbs energy from an exothermal reaction on the catalytically active surface much faster than the exhaust gas flowing over it. The temperature measurement therefore takes place at the heat source, so to speak, and during transient processes reacts considerably faster and more accurately than the gas temperature in many cases. However, the gas temperature shows similar reactions, although delayed and/or weakened, so that measurements which can be evaluated are not only possible to be taken at the structure, but within limitations also inside the catalytic converter in the exhaust gas itself.
A measuring method is furthermore known from German Published, Non-Prosecuted Application DE 41 00 397 A1, corresponding to U.S. Pat. No. 5,133,184, wherein the condition of an internal combustion engine is briefly disrupted, particularly by turning off the ignition and injecting gasoline during operation under load, in the course of which the reaction of temperature sensors at various places in the exhaust system is observed. In this connection it is also suggested to differentiate the temperature signals and to store the differentiated signals or to compare those signals with already stored previous signals.
The following basic problems occur during temperature measurement in the exhaust system, in particular inside catalytic converters: On one hand, the occurring absolute temperatures are so high that it is impossible to measure the absolute temperatures with sufficient accuracy with conventional, economically usable temperature sensors in order to draw dependable conclusions therefrom. In particular, when forming the difference between several temperature sensors, the measuring errors are cumulative so that because of the measuring errors it is no longer possible to detect the changes, which are small per se, with certainty. A further problem lies in the fact that the values measured by a temperature sensor inside a catalytic converter depend to a large extent on the location where the sensor has been installed. If, for example, a temperature sensor is installed close to the beginning of an approximately 20 cm long catalytic converter, after a short time in operation this sensor can be located in a catalytically less active zone, so that the values measured there can provide practically no further information regarding the operational ability of the remaining catalyst volume. On the other hand, during transient processes, particularly during brief disruptions of the operational condition of the internal combustion engine with a catalyst which has only been briefly operational, a temperature sensor installed at the end of the catalytic converter would measure practically no change at all, since the entire exothermal reaction occurs in the front portion of the catalytic converter, so that brief changes cannot be detected at all or can only be detected belatedly and are greatly weakened at the end of the catalytic converter, because of the heat capacity of the portion located behind it. The situation becomes even more difficult in connection with the frequent cases wherein a catalytic converter is formed of several partial bodies disposed one behind the other. In this case the meaningfulness of a measured temperature value obtained at any arbitrary location is very small in relation to the total condition of the system.